1. Field of the Invention
This invention relates to a cable and pulley arrangement that provides for minimal slippage of the cable about the pulley. It is particularly well suited for applications requiring accurate and repeatable movements of devices attached to such pulleys, such as motorized and automated Pan/Tilt/Zoom Cameras with predefined fields of view.
2. Discussion of Related Art
Pulley and Cable arrangements are often used to control the movement of a device connected to one pulley via the controlled movement of another pulley. For example, a security camera's field of view can be adjusted in the horizontal or vertical direction by controlling one or more motors that are connected by pulley and cable to a movable platform upon which the camera is attached. Another example is the manual rotation of a wheel to effect the rotation of another device, such as the rudder of a boat.
To maintain a correspondence between the rotation of the driving pulley and the driven pulley, the cable which is routed about the pulleys is maintained under tension, and the resultant friction forces a relatively fixed relation between each pulley and the cable. Often, due to the nature of the construction of the cable, for example as a braided filament of finer strands, the frictional forces are not equivalent in both directions of rotation of the pulleys. This can result in a gradual creeping of the cable in the direction of lesser frictional force, such that the correspondence between the driving and driven pulleys is no longer maintained. To reduce the slippage in half, traditional devices often comprise a cable that is fixedly attached to one pulley, and frictionally held to the other. Fixedly attaching the cable to both pulleys is not effective, for it introduces hysteresis, or backlash, as the cables stretch over time and the driving pulley must initially take up the slack before a movement occurs on the driven pulley. Cable stretching also allows for overshoot, as the inertia of the driven object causes it to continue to rotate when the driving motor stops. To compensate for cable stretch, the driving and driven pulleys are typically maintained under high tension, using for example, spring mounted pulleys.
Alternatively, to assure a proper and repeatable correspondence between the rotation of the driving pulley and the driven pulley, a modified pulley and cable arrangement may comprise a toothed wheel and a chain. When the driving pulley is rotated, the chain holds an absolute position relative to the wheel because of the interlocking of the chain and the teeth of the wheel. Such a chain driven apparatus, however, typically exhibits the aforementioned hysteresis, or backlash. To reduce the hysteresis caused by the chain linkage, hybrid devices typically comprise a chain and toothed gear on the driving pulley, and a fixedly attached cable on the driven pulley.
To overcome the hysteresis of a chain, or the slippage of a filament, or both, the static tension on the chain or cable must be high. To accommodate the forces produced by the high tension between pulleys, the supporting structures, holding the pulleys apart and maintaining the tension, must be reinforced. This reinforcement adds additional weight and cost to the apparatus, a weight and cost which could be reduced by minimizing the need for high tension between the pulleys.